Ball hole welding using the friction stir welding (fsw) process

ABSTRACT

A roller cone drill bit includes a bit body, at least one leg extending downward from the bit body, a journal on each leg, and a roller cone mounted on each journal. A ball race is configured between each journal and roller cone, and a plurality of retention balls is disposed within each ball race. A ball hole extends from the back face of each leg to the ball race, and a ball hole plug fits within the ball hole. The ball hole plug is secured to the leg by a friction stir weld.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit pursuant to 35 U.S.C. §120 as acontinuation-in-part application of U.S. patent application Ser. No.11/136,609, filed on May 23, 2005, which claims priority to all of thesubject matter included in provisional applications with Ser. No.60/573,707, filed May 21, 2004, Ser. No. 60/637,223, filed Dec. 17, 2004and Ser. No. 60/652,808, filed Feb. 14, 2005, and non-provisionalapplications with Ser. No. 11/090,909, filed Mar. 24, 2005 and Ser. No.11/090,317, filed Mar. 24, 2005. The above referenced applications arehereby incorporated by reference in their entirety.

BACKGROUND OF INVENTION

1. Field of the Invention

Embodiments disclosed herein relate generally to roller cone drill bitsused in wellbore operations. In particular, embodiments disclosed hereinrelate generally to ball hole plugs and methods of welding ball holeplugs to roller cone drill bits using friction stir welding.

2. Background Art

Historically, there have been two main types of drill bits used drillingearth formations, drag bits and roller cone bits. The term “drag bits”refers to those rotary drill bits with no moving elements. Drag bitsinclude those having cutters attached to the bit body, whichpredominantly cut the formation by a shearing action. Roller cone bitsinclude one or more roller cones rotatably mounted to the bit body.These roller cones have a plurality of cutting elements attached theretothat crush, gouge, and scrape rock at the bottom of a hole beingdrilled.

Roller cone drill bits typically include a main body with a threaded pinformed on the upper end of the main body for connecting to a drillstring, and one or more legs extending from the lower end of the mainbody. Referring to FIG. 1, a conventional roller cone drill bit,generally designated as 10, consists of bit body 11 forming an upper pinend 12 and a cutter end of roller cones 13 that are supported by legs 14extending from body 11. Each leg 14 includes a journal 15 extendingdownwardly and radially inward towards a center line of the bit body 11,with cones 13 mounted thereon. Each of the legs 14 terminate in ashirttail portion 16. The threaded pin end 12 is adapted for assemblyonto a drill string (not shown) for drilling oil wells or the like.

Conventional roller cone bits are typically constructed from at leastthree segments. The segments are often forged pieces having an upperbody portion and a lower leg portion. The lower leg portion is machinedto form the shirttail section and the journal section. Additionally,lubricant reservoir holes, jet nozzle holes, and ball races are machinedinto the forgings. Roller cones are rotatably mounted to a bearingsystem on the formed journals, and the leg segments are positionedtogether longitudinally with journals and cones directed radially inwardto each other. The segments may then be welded together usingconventional techniques to form the bit body. Upon being weldedtogether, the internal geometry of each leg section forms a center fluidplenum. The center fluid plenum directs drilling fluid from the drillstring, out nozzles to cool and clean the bit and wellbore, etc.

Roller cone bits may use a roller bearing system, a journal bearingsystem, or a combination of the two to allow rotation of the rollercones about the journal. Each type of bearing system is ordinarilycomprised of a number of separate components, including primarybearings, secondary bearings, a seal system, features that resist thrustloading, and a lubrication system. Also typical to both bearing systemsare cone retention balls, which are used to prevent roller cones fromseparating from their journals.

Generally, roller bearing systems use rollers to separate the rollercones from the journal. A cone retention ball bearing is usuallyprovided to carry axial load, and the rollers typically carry radialloads. Journal bearing systems, on the other hand, use a film oflubricant to separate the roller cones from the journal. The innersurfaces of roller cones are specially designed so the film of thelubricant prevents contact between the roller cone and journal. Rollerbearings are common in roller cone drill bits, especially in roller conedrill bits with diameters larger than twelve inches, because they canreliably support large loads and generally perform well in the drillingenvironment. Bits having small diameters commonly use journal bearingsystems because there is less space to install suitably sized rollers ina small cone.

Referring to FIG. 2, a typical ball bearing system is shown within aroller cone drill bit leg. Roller bearings 201 are placed around thejournal 202 prior to sliding the journal 202 into the roller cone body203. Alternate bearing systems may be used to separate the roller conebody 203 from the journal 202, such as floating bearings or a journalbearing system. The journal 202 has a journal race surface 204, and theroller cone body 203 has a roller cone race surface 205, which meets toform a ball race 206. A ball hole 207 extends from the back face 208 ofthe drill bit leg 209 to the journal race surface 204. A plurality ofcone retention balls 210 are then inserted through the ball hole 207into the ball race 206, to hold the roller cone 203 on the journal 202.Once the balls 210 are in place, a ball hole plug 211 is inserted intothe ball hole 207 and welded into place, to prevent the roller cone 203from slipping off the journal 202.

To prevent damage to the cone retention balls 210 and edges of the ballhole 207, cutter designs known in the art have the ball hole 207 placedat 180 degrees from the load bearing zone of the journal 202. Thisplacement is selected to prevent forcing the balls 210 against the roughedges of the ball hole 207 as they pass over the hole 207. If the ballhole 207 were positioned in the load bearing zone, the balls 210 wouldforcibly impact the edges of the ball hole 207, probably resulting inmetal chips and debris being removed from the journal 202 so as tocontaminate the lubricant and eventually destroy the bearings and seals.

Contained within the bit body is a grease reservoir system (not shown).A lubricant passage 212 is provided from the reservoir to race surfaces204, 205 formed between the journal 202 and roller cone body 203, tolubricate race surfaces 204, 205 by a lubricant or grease composition.Lubricant or grease also fills the portion of the ball hole 207.Lubricant or grease is retained in the bearing structure by a resilientseal 213 between the roller cone 203 and journal 207.

For many applications, roller cone drill bits are limited by the bearingcapacity or bearing life of the bit. A contributing cause of bearingfailure in roller cone systems is failure of the weld joint between theball hole plug and the back face of the leg. In addition to providing asecure weld, protection of the weld joint from wear, erosion andcorrosion is necessary to prevent failure of the plug and/or leg in theplug region, and ultimately, failure of the bit.

Current methods of welding the ball hole plug to the back surface of thebit leg are difficult to implement and may cause flaws in the weldjoint. For example, GMAW welding can cause porosity, inclusions, cracksand an area of un-fused material at the weld root, any of which can leadto premature failure by initiating fatigue stresses. Further, in gas orplasma arc welding, heat of the arc weld and molten weld deposit canpotentially affect the seal integrity of the weld joint. Additionally,dissimilar chemistry in a deposit weld metal and the leg steel may causegalvanic corrosion in caustic or acidic drilling conditions.

Another cause of bearing failure in roller cone drill bit systems isspalling, which may occur, for example, when the ball hole plug is notexactly in line with the journal race surface and continuously passingretention balls flake off material from the plug. When the surfacespalls, debris contaminates the lubricant which causes rapid wear anddamage to the rest of the operable bearing and seal components whicheventually results in bearing failure. Accordingly, there exists acontinuing need for developments in securing a ball hole plug to a bitleg that may at least provide for increased bearing life.

SUMMARY OF INVENTION

In one aspect, embodiments disclosed herein relate to a roller conedrill bit including a bit body, at least one leg extending downward fromthe bit body. Each leg includes a leg back face and a journal and eachjournal has a journal race surface, a roller cone mounted on eachjournal, wherein each roller cone includes a roller cone race surface, aball race configured between the journal race surface and the rollercone race surface, a plurality of retention balls disposed within theball race, a ball hole extending from the leg back face to the journalrace surface, and a ball hole plug. The ball hole plug is secured to theleg by a friction stir weld.

In another aspect, embodiments disclosed herein relate to a roller conedrill bit including a bit body and at least one leg extending downwardfrom the bit body. Each leg includes a leg back face and a journal andeach journal has a journal race surface. A roller cone is mounted oneach journal, wherein each roller cone has a roller cone race surface. Aball race is configured between the journal race surface and the rollercone race surface, and a plurality of retention balls is disposed withinthe ball race. A ball hole extends from the leg back face to the journalrace surface, wherein the ball hole is non-cylindrical near the leg backface. A ball hole plug is secured to the leg by a friction stir weld.The ball hole plug includes a plug head, wherein the plug head isnon-cylindrical, a plug body, and a ball retainer end.

In another aspect, embodiments disclosed herein relate to a method forretaining a roller cone on a bit leg, including mounting a roller coneon a journal extending downward from the bit leg, inserting a pluralityof retention balls into a ball hole extending through a leg back face tothe journal, inserting a ball hole plug into the ball hole, and frictionstir welding the ball hole plug to a back face of the bit leg.

In yet another aspect, embodiments disclosed herein relate to a methodfor retaining a roller cone on a bit leg, including inserting aplurality of retention balls into a ball hole, inserting a ball holeplug into the ball hole, friction stir welding the ball hole plug to aback face of the bit leg using a friction stirring tool, and removingthe friction stirring tool.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conventional roller cone drill bit.

FIG. 2 is a conventional ball bearing system of a roller cone drill bit.

FIGS. 3A and 3B show a ball hole plug friction stir welded to a bit leg.

DETAILED DESCRIPTION

Generally, embodiments disclosed herein relate to securing a ball holeplug to a roller cone drill bit leg. In particular, embodimentsdisclosed herein relate to securing the ball hole plug using frictionstirring. Embodiments of the present disclosure related to securing theball hole plug may also relate to improvements in assembly of the ballhole plug into the ball hole by fitting a shaped plug into acorresponding shaped ball hole prior to friction stirring.

Friction stirring is a process by which frictional heat plasticizes,mixes, and forges metal, metal alloys, and other materials. Frictionstirring uses a combination of rotational and directional motion appliedto the surface of an object to be treated. A rotating member isconventionally applied to the surface that is to be friction stirred andis moved in a particular direction until a plasticized state of thematerial is achieved. The rotating member is moved along the surface totreat the material by changing the material microstructure. Frictionstirring includes friction stir processing, friction stir mixing, andfriction stir welding (FSW). Friction stir processing is a treatmentprocess, which generally involves engaging two or more previouslyadjoined materials (i.e., previous weld) to strengthen or improve theweld characteristics. Alternatively, friction stir processing may referto treating a single material of a workpiece. FSW involves engaging twoor more adjoining materials to form a weld.

In one embodiment of the present disclosure, as shown in FIGS. 3A and3B, a ball hole plug 311 is friction stir welded to the leg back face309. A tool used for friction stirring is characterized by a generallycylindrical tool 300 having a shoulder 301 and a pin 302 extendingdownward from the shoulder. The pin 302 is rotated as force is exertedto urge the pin 302 and a workpiece 330 together. The workpiece 330, inFIG. 3A, includes the ball hole plug 311, the back face of the leg 309,and an interface 360 between the ball hole plug 311 and the leg 309.Frictional heating caused by the interaction between the rotating pin302 and the workpiece 330 causes the workpiece material to softenwithout reaching the melting point of the material, which results inplasticization of the workpiece material. Once sufficient heat isgenerated, the pin 302 is plunged into the workpiece 330 through theinterface 360. The tool 300 is then moved along the workpiece 330,plasticizing the workpiece material as it flows around the pin 302. Thefriction stirring tool 300 is moved along the interface 360 in such amanner that the pin 302 presses into the interface 360 at an orientationthat is co-planar with the interface 360 between the two materials. Theresult is a solid state bond 370 between the ball hole plug 311 and theleg 309. Friction stir welding does not require a solder or fillermaterial to form a bond, but the use of an additional material is notnecessarily outside the scope of the present invention. Additionalmaterial may be used, for example, to add corrosion inhibitors, wearresistant material, and other material enhancing properties.

The resulting solid state bond of a friction stir weld is aninter-metallic atomic bond formed by mechanical deformation. Asolid-state bond differs from bonds formed by conventional weldingtechniques (i.e., welds resulting in a fusion bond or solder or brazebond) in that conventional welding techniques include melting thewelding material and then cooling the material to form a bond. The highrates of heating and cooling during conventional welding may result innon-uniformity throughout the microstructure of the welded material,which may create different strain rates and increased stress within thewelded material. A solid state bond, on the other hand, does not requirethe workpiece material to melt. Thus, more uniformity of themicrostructure, and better mechanical properties of the welded materialmay be achieved. For example, a solid state bond may have substantiallyno metallurgical discontinuities, including minimal or no porosity.

Referring back to FIGS. 3A and 3B, the ball hole plug 311 comprises aplug head 312, a plug body 313, and a ball retainer end 314. The plughead 312 comprises a top surface 316 and a side surface 317. The plughead top surface 316 is flush with the back face surface of the leg 309,and the plug head side surface 317 fits against the ball hole wall 307,at the opening portion 308 of the ball hole, to create the interface 360between the ball hole plug 311 and the leg 309. However, the plug head312 may initially protrude from the back face surface of the leg 309 inorder to have a flush surface with the leg back face after frictionstirring. The ball retainer end 314 has a concave surface 315 with aradius of curvature that mates with a corresponding radius of curvatureof the cone retention balls 310.

Generally, ball hole plug heads 312 are cylindrical in shape. However,it is within the scope of the present disclosure that the plug head 312may be cylindrical or non-cylindrical in shape. When the plug head 312is cylindrical in shape, it may be difficult to orient the ball holeplug 311 in such a manner that the plug retainer end 314 configuresexactly to the cone retention balls 310. However, in accordance withvarious embodiments of the present disclosure, the ball hole 307 isformed in a shape corresponding with a non-cylindrical plug head 312.The corresponding shapes are oriented in a position that secures theretainer end 314 in configuration with the cone retention balls 310 whenthe plug head 312 is inserted into the ball hole 307.

Any non-cylindrical shaped plug head and corresponding ball hole iswithin the scope of the present disclosure, including, for example, anoval-shaped plug head, a plug head with at least one flat side, atriangular-shaped plug head, a rectangular plug head, etc. Additionally,the plug head may be non-symmetrical in shape, such that the plug headhas a notch, a protrusion, or other variation from the general shape ofthe plug head.

As shown in FIG. 3A, the pin 302 may be plunged to a depth in theworkpiece 330 such that part of the interface 360 is friction stirwelded. Alternatively, as shown in FIG. 3B, the pin 302 may havesufficient depth so as to friction stir weld the entire height of theball hole plug head 312. Friction stir welding the workpiece such thatthe entire plug head 312 is completely consumed may yield a reducedsubsurface notch affect at the weld root. A reduced subsurface notchaffect may be desirable because the notch affect can promote failure byallowing fatigue crack initiation sites. However, depending on thegeometry of the plug head, the entire head may not be consumed. Forexample, rather than friction stirring the entire plug head 312, onlythe joint interface 360 around the plug head may be friction stirred.Furthermore, it is within the scope of the present disclosure that thediameter of the pin 302 may be smaller, larger, or equal to the diameterof the plug head 312. Likewise, the shoulder 301 may be smaller, larger,or equal to the diameter of the plug head 312.

Large forces may be exerted between the pin and the workpiece in orderto apply sufficient pressure to the workpiece to cause plasticization ofthe material. For example, for friction stir welding an aluminum alloyworkpiece of ¼-inch thickness, forces of up to 4000 pounds or more mayhave to be exerted between the pin and the workpiece. Where theworkpieces have sufficient structural strength and rigidity, some of theforce may be absorbed by the workpieces themselves.

Furthermore, FIGS. 3A and 3B show a friction stirring tool 300positioned at an orientation that is co-planar with the interface 360between the ball hole plug head 312 and the leg 309. However, inaccordance with another embodiment of the present disclosure, thefriction stirring tool may be moved along an interface in such a mannerthat the pin is oriented perpendicular to the interface plane. Forexample, a layer of wear resistant material may be applied to the outersurface of a drill bit, thereby creating an interface perpendicular tothe pin. Depending on the component being friction stirred and itsconfiguration, one skilled in the art would appreciate that eitherorientation of the tool may be used.

The ball hole plug and the bit leg may be formed from the same material,or alternatively, they may be formed from dissimilar material. Further,the ball hole plug may be formed from material with a higher yieldstrength and toughness than the leg material. The ball hole plug and legmay be formed from material selected from, for example, at least one ofthe following: austenitic steel, carbon steels, low alloy carburizingsteel, high alloy carbon steel, and high alloy materials. High alloymaterials include, for example, iron-, cobalt-, or nickel-basedmaterials, which may be used for higher strength or improved corrosionresistance. Additionally, the ball hole plug material may be subjectedto different processing conditions than the leg material. For example,the ball hole plug material may be annealed or heat treated to have thesame hardness as the leg material.

Furthermore, additional material may be added to the friction stirringprocess, so as to control mechanical properties of the resultingworkpiece material, including one or more of the following uniqueproperties: improved corrosion resistance, higher toughness orequivalent toughness, higher hardness, fatigue resistance, crackresistance, minimal or no significant heat affected zone, and higheryield strength and wear resistance than the base material used in adrill bit. In one embodiment of the present disclosure, an additivematerial is friction stirred into the roller cone drill bit leg,including over the ball hole plug weld to increase wear resistance. Forexample, an additive material may be applied by conventional methods tohardface the outer surface of a drill bit. The hardfacing may then betreated using the friction stirring methods disclosed herein, dependingon the desired material properties for the particular application, suchas hardness, toughness, casing-friendly wear resistance, etc.

Additive material may include, for example, metal matrix composites,ferrous alloys such as steel and stainless steel, non-ferrous materialssuch as aluminum, aluminum alloys, and titanium, super alloys such asnickel, iron-nickel, and cobalt-based alloys generally suitable for useat temperatures above 1,000 degrees Fahrenheit, and air hardened steels.These materials may be described as “high melting temperaturecompounds,” or compounds having a melting temperature greater thansteel. Additional elements in the types of materials that may befriction stirred include, but are not limited to, diamond, tungstencarbide, chromium, molybdenum, manganese, silicon, carbon, boron,tungsten, aluminum, titanium, niobium, tantalum, vanadium, nickel,cobalt, zirconium, phosphorus, and rhenium.

Additive materials may be applied to the back face of a drill bit leg,including over the ball hole plug weld by any means known in the art, asdescribed in U.S. patent application Ser. No. ______ (Attorney DocketNumber 05516/446001), which is filed concurrently herewith and isincorporated by reference in its entirety. For example, additivematerial may be applied as hard particles, as a tape, or as a plate tothe leg base material prior to friction stirring. Methods of applicationinclude: thermal spraying, plasma spraying, using adhesives to bind thefriction stirring material to the base material, entrenching a packedpowder into the surface of the base material, sandwiching a firstfriction stirring material between the base material and a secondfriction stirring material, etc.

Alternatively, the additive material may have been welded to a basematerial using a variety of conventional techniques, such as GMAW (gasmetal arc welding), GTAW (gas tungsten arc welding), PTA (plasmatransferred arc), FCAW (flux cored arc welding), etc. Due to the phasetransformations (to liquid state, then cooled to a solid state) thatoccur during such conventional techniques, the microstructure canpossess undesirable characteristics, such as precipitation of unwantedphases or structures, grain growth, and residual stresses. Thus, one ormore thermal treatments may have been performed on the welded material(including pre- and/or post-heat treatments) to relieve some of thoseresidual stresses and minimize cracking. In accordance with embodimentsof the present disclosure, the additive material may subsequently befriction stir processed to achieve an improved fine-grainedmicrostructure (with improved material properties).

In one embodiment of the present disclosure, a plate may be frictionstir welded to the back surface of a drill bit leg and cover the ballhole. The plate may comprise nickel or stainless steel alloys, highstrength steel alloys, or any air hardenable steel, including D2 and A2steel, or alloy steels such as 4815, 9313, and 8720 steels. In such anembodiment, the ball hole plug may be welded (by conventional means orby friction stir welding) to the leg prior to friction stir welding theplate to the leg, or alternatively, the ball hole plug and the plate maybe friction stir welded to the leg during a single friction stirringprocess. However, while the leg may be friction stirred prior to orafter assembly of the drill bit, the ball hole plug must be weldedbefore the drill bit, in particular the multiple leg forgings, isassembled. Thus, if a plate is to be friction stir welded after assemblyof the drill bit, the ball hole plug must have been welded to the legprior to welding the plate to the leg.

Friction stir welding typically leaves lower asperity heights andresults in a smoother finish than conventional welding techniques.However, a friction stirred surface may have a depressed surface height,i.e., a keyhole, at the location where the friction stirring tool wasremoved from the workpiece. Depending on the application of theworkpiece being friction stirred, a keyhole may be left in theworkpiece, the keyhole may be filled, or the keyhole may be diminishedby certain tool removal processes. In one embodiment of the presentdisclosure, a keyhole is left in the ball hole plug material uponremoval of the friction stirring tool. In another embodiment of thepresent disclosure, a gradual removal process is used to minimize theoccurrence of a keyhole at the point of exit. The gradual removalprocess includes: beginning the removal of the friction stirring tool atan initial location in the workpiece; gradually pulling the frictionstirring tool out of the workpiece as the tool is moved a distance awayfrom the initial removal location; and finally, completely removing thefriction stirring tool at a distance from the initial position. Theremoval process may also be aided by use of a secondary, sacrificialmaterial onto which the friction stirring tool may be pulled, tominimize the effect of the tool removal on the leg.

Using the friction stir treatment methods of the present disclosure, thesolid-state processing principles associated with friction stirring, maylikely reduce the microstructure defects present in the original weld ordeposit, reducing the incidence of cracking. By reducing the incidenceof cracking, the need for additional heat processing treatments, such aspre- and/or post-heat treatments may be eliminated. Moreover, theprocessing technique may be less hazardous, which may also allow forfriction stirring at any given location, including at the rig site,allowing for better rebuild service. Another byproduct of the frictionstirring techniques of the present disclosure may be a reduction in thesurface roughness, i.e., reduced asperity heights, as compared to aconventional weld. Lower asperity heights result in a smoother finish,which reduces an apparent need for surface finishing or grinding.

In addition to the above mentioned benefits of friction stirring overconventional welding techniques, a greater hardness of the frictionstirring material may be achieved without losing toughness.Specifically, friction stirring results in materials having a refinedgrain microstructure. Refined grain microstructures provide the frictionstirred material with both increased toughness and increased strength,as well as increased corrosion resistance, and other favorable materialcharacteristics. Conventional welding, on the other hand, generallyresults in materials having an inverse relationship between strength andtoughness (toughness decreases as strength is increased).

Increased hardness depends on the material composition and type ofmaterial being friction stirred. The bit leg material is generally madeof low alloy carburizing steels, such as 4815, 8720, 4718, and 9313.However, other materials, such as 4130, 4145, and other alloy steels,may be used as bit leg material. Friction stirring 4815 steel that hasbeen heat treated to have a hardness of 36-40 HRc may yield a hardnessincrease of 5-10 HRc. However, friction stirring 4140 steel or 4130steel, for example, may result in an increased hardness of 20 HRc ormore. Such improved hardness may result from the change in the materialmicrostructure (i.e., through grain refinement/recrystallization toproduce fine precipitates such as carbides). Further, friction stirwelding a ball hole plug to a bit leg may result in the weld strengthbeing higher than the strength of the parent material (the originalmaterial being friction stir welded).

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A roller cone drill bit, comprising: a bit body; at least one legextending downward from the bit body, wherein each leg comprises a legback face and a journal and each journal has a journal race surface; aroller cone mounted on each journal, wherein each roller cone has aroller cone race surface; a ball race configured between the journalrace surface and the roller cone race surface; a plurality of retentionballs disposed within the ball race; a ball hole extending from the legback face to the journal race surface; and a ball hole plug, wherein theball hole plug is secured to the leg by a friction stir weld.
 2. Theroller cone drill bit of claim 1, wherein the ball hole plug is weldeddirectly to the leg back face without a filler material.
 3. The rollercone drill bit of claim 1, wherein the ball hole plug material is thesame as the leg material.
 4. The roller cone drill bit of claim 1,wherein the ball hole plug comprises a material selected from at leastone of austenitic stainless steel, high alloy carbon steel, and nickeland cobalt based alloys.
 5. The roller cone drill bit of claim 3,wherein the high alloy materials are nickel based materials.
 6. Theroller cone drill bit of claim 1, wherein the ball hole plug comprisesthe same material as the leg.
 7. The roller cone drill bit of claim 1,wherein the ball hole plug comprises a different material from the leg.8. The roller cone drill bit of claim 7, wherein the ball hole plugcomprises material with a higher yield strength and higher toughnessthan the leg.
 9. The roller cone drill bit of claim 1, wherein the ballhole plug is heat treated.
 10. The roller cone drill bit of claim 1,further comprising a keyhole in the friction stir weld.
 11. The rollercone drill bit of claim 1, wherein no keyhole remains in the frictionstir weld.
 12. The roller cone drill bit of claim 1, wherein the ballhole plug comprises: a plug head, wherein the plug head comprises a topsurface and a side surface; a plug body; and a ball retainer end. 13.The roller cone drill bit of claim 12, wherein the top surface of theplug head is flush with the leg back face.
 14. The roller cone drill bitof claim 12, wherein the friction stir weld extends down the entire sidesurface of the plug head.
 15. The roller cone drill bit of claim 12,wherein the friction stir weld extends down to a depth of the sidesurface.
 16. The roller cone drill bit of claim 12, wherein the plughead is non-cylindrical.
 17. The roller cone drill bit of claim 16,wherein the ball hole is non-cylindrical at the opening to the leg backface.
 18. A roller cone drill bit, comprising: a bit body; at least oneleg extending downward from the bit body, wherein each leg comprises aleg back face and a journal and each journal has a journal race surface;a roller cone mounted on each journal, wherein each roller cone has aroller cone race surface; a ball race configured between the journalrace surface and the roller cone race surface; a plurality of retentionballs disposed within the ball race; a ball hole extending from the legback face to the journal race surface, wherein the ball hole isnon-cylindrical near the leg back face; and a ball hole plug, whereinthe ball hole plug is secured to the leg by a friction stir weld, theball hole plug, comprising: a plug head, wherein the plug head isnon-cylindrical; a plug body; and a ball retainer end.
 19. The rollercone drill bit of claim 19, wherein the ball retainer end has a matingcurve with the same radius of curvature as the race.
 20. A method forretaining a roller cone on a bit leg, comprising: mounting a roller coneon a journal extending downward from the bit leg; inserting a pluralityof retention balls into a ball hole extending through a leg back face tothe journal; inserting a ball hole plug into the ball hole; and frictionstir welding the ball hole plug to a back face of the bit leg.
 21. Themethod of claim 21, further comprising providing an additive materialand friction stir mixing the additive material into the bit leg.
 22. Themethod of claim 21, further comprising covering the ball hole plug witha plate prior to friction stir welding, wherein the friction stirwelding welds the ball hole plug, the plate, and the bit leg together.23. The method of claim 23, wherein the ball hole plug is friction stirwelded to the bit leg prior to friction stir welding the plate.
 24. Themethod of claim 21, wherein the ball hole plug comprises anon-cylindrical head and the ball hole is non-cylindrical near the backface of the bit leg.
 25. The method of claim 21, further comprisingwelding a plurality of bit legs together.
 26. A method for retaining aroller cone on a bit leg, comprising: inserting a plurality of retentionballs into a ball hole; inserting a ball hole plug into the ball hole;friction stir welding the ball hole plug to a back face of the bit legusing a friction stirring tool; and removing the friction stirring tool.27. The method of claim 27, further comprising removing the frictionstirring tool at an initial removal location.
 28. The method of claim27, further comprising removing the friction stirring tool by a gradualremoval process, wherein the gradual removal process comprises:beginning the removal of the friction stirring tool at an initialremoval location; gradually removing the friction stirring tool as thefriction stirring tool is moved a distance away from the initial removallocation; and completely removing the friction stirring tool at adistance from the initial removal location.
 29. The method of claim 27,wherein removing the friction stirring tool comprises pulling thefriction stirring tool onto a sacrificial material.